You can do anything in your task such as network requests, timers and so on, as long as it doesn't touch UI. Once your task completes (i.e. the promise is resolved), React Native will go into "paused" mode (unless there are other tasks running, or there is a foreground app).

The Java API

Yes, this does still require some native code, but it's pretty thin. You need to extend HeadlessJsTaskService and override getTaskConfig, e.g.:

Caveats

By default, your app will crash if you try to run a task while the app is in the foreground. This is to prevent developers from shooting themselves in the foot by doing a lot of work in a task and slowing the UI. You can pass a fourth boolean argument to control this behaviour.

If you start your service from a BroadcastReceiver, make sure to call HeadlessJsTaskService.acquireWakeLockNow() before returning from onReceive().

Example Usage

Service can be started from Java API. First you need to decide when the service should be started and implement your solution accordingly. Here is a simple example that reacts to network connection change.

Following lines shows part of Android manifest file for registering broadcast receiver.

Broadcast receiver then handles intent that was broadcasted in onReceive function. This is a great place to check whether your app is on foreground or not. If app is not on foreground we can prepare our intent to be started, with no information or additional information bundled using putExtra (keep in mind bundle can handle only parcelable values). In the end service is started and wakelock is acquired.